The present invention relates generally to microwave monolithic integrated circuits, and more particularly, to a microwave monolithic integrated circuit fabrication and test method and test circuits that determine the performance potential of the circuits during the manufacturing process.
Currently, the cost to produce a GaAs microwave monolithic integrated circuits is $5,000 to $10,000 per wafer. However, not all completed circuits produce acceptable RF performance. Transistors are RF testable as soon as the basic transistor structure is completed, which is 30-40% of the way through the manufacturing process. Currently RF tests are performed on completed wafers. The conventional method for testing completed wafers is to evaluate completed microwave monolithic integrated circuits using on-wafer RF probes coupled to a 50 ohm measurement system.
Two methods are commonly used to assess performance potential of microwave monolithic integrated circuit wafers. DC tests can be performed early in the process sequence (at the same point as the present invention implements RF tests). However, the correlation between the DC tests and RF results is not good. Therefore, the risk of rejecting "RF good wafers" or processing "RF bad" wafers based solely on DC test results is high. An alternative is to perform tests after completion of wafer processing. The problem with this approach is that significant value has been added to wafers that could have been tested earlier and found to be "RF bad". Any funds expended on wafers known (or even unknown) to be "RF bad" are wasted since the wafer costs are unrecoverable after processing is completed.
The current approaches to assessing performance potential early in the process sequence have been to take DC measurements of transistor current-voltage characteristics. Unfortunately, the correlation between these measurements and the RF performance of the completed circuits is relatively poor at this stage of the technology evolution. An alternative technique which has been proposed is to measure the circuit performance at some frequency well below its intended operating frequency; for example, 2 GHz for a circuit intended for 10 GHz operation. Again, the correlation between low frequency test results and higher frequency performance is not as good as desired.
Therefore, it is desirable to measure the performance potential of a wafer as soon as possible in the process so that "RF bad" wafers are not processed to completion. A significant savings would result if the performance potential of microwave monolithic integrated circuits could be assessed early in the manufacturing process.